1. Field of the Invention
The present invention relates to a photoconductor that comprises a photoconductive layer, wherein the photoconductive layer comprises a charge generating layer, a charge transporting layer, and a crosslinked charge transporting layer, and wherein these layers are laminated in order on a substrate; and a image forming process, a image forming apparatus, and a process cartridge that utilize the photoconductor respectively.
2. Description of the Related Art
Information processing systems based on electrophotographic process have been dramatically developing. In particular, laser printers and digital copiers, which convert information into digital signals and record the information through optical technologies, have been remarkably improved in their printing quality and reliability. These laser printers and digital copiers are still demanded higher image quality, higher speed, and more compacted size.
In addition, recently, the market of laser printers and digital copiers capable of full-color printing has been increasing rapidly. Such full-color printing requires duplicating toner images of at least four colors, therefore, the technologies as to higher speed and more compacted size are needed still more. In order to achieve the higher speed and more compacted size, the photoconductors employed to the laser printers and digital copiers (hereinafter, sometimes referring to “electrophotographic photoconductor” or “electrostatic latent image carrier”) should be improved the sensitivity and also should be miniaturized.
Provided that the conventional photoconductors are employed as they are, the exchanging periods will remarkably shorter since the conditions in use are more sever. Accordingly, improvements in resistance and stability of photoconductors employed in such apparatuses are absolutely required in order to achieve higher speed and more compacted size of apparatuses.
The resistance of photoconductors is evaluated based on the image qualities; specifically, laser printers and digital copiers that make use of reversible developing are mainly evaluated the life based on the background smear, which is numerous number of black points printed on white media. Accordingly, in order to achieve the higher speed and more compacted size of the apparatuses, the life of photoconductors should be prolonged along with higher sensitivity of photoconductors and controlling of the background smear.
In order to achieve the more compacted size of the apparatuses, a charge generating substance with higher quantum efficiency is absolutely necessary.
As for the organic photoconductors with higher sensitivity, titanyl phthalocyanines are widely and effectively utilized that show a maximum diffraction peak at 27.2 degrees as Bragg 2θ angle under CuK-α characteristic X-ray wavelength at 1.542 angstroms.
However, the photoconductors formed from the titanyl phthalocyanines are significantly susceptible to the background smear due to pigment flocculation or decreased charging property. In particular, the background smear is a serious matter as described above. Therefore, apparatuses with photoconductors formed from titanyl phthalocyanines are not establish both of the higher speed and more compacted size due to poor image stability, since the effect of the background smear is significant even if the higher speed is attained (see Japanese Patent Application Laid-Open (JP-A) No. 2001-19871).
On the other hand, protective layers on photoconductors are known to be effective to enhance the abrasion resistance thereby to prolong the life of the photoconductors. The background smear, which determines the life of photoconductors, is enhanced by the fatigue or abrasion of photoconductors under repeated usages. The control of the photoconductor abrasion under repeated usages by means of a protective layer on the photoconductor surface may lead to the decrease of background smear, through the control of the electric field increase derived by the abrasion, thereby may be a very effective way to prolong the life.
The techniques to improve abrasion resistance of the photoconductive layer include (1) using a curable binder in the surface layer (for example, JP-A No. 56-48637), (2) using a polymer charge transport material (for example, JP-A No. 64-1728), (3) dispersing an inorganic filler in the surface layer (for example, JP-A No. 4-281461) and the like. Among these techniques, the use of a curable binder in (1) tends to cause reduction in image density since the curable binder has poor compatibility with the charge transporting material and impurities such as a polymerization initiator and unreacted residue is likely to increase the residual potential. Also, the use of a polymer charge transport material in (2) may somewhat improve the abrasion resistance. However, it is not sufficient for satisfying the durability required in the organic photoconductor. Further, it is difficult to polymerize and purify the polymer charge transporting material. Thus, it is impossible to obtain it at high purity and to attain stable electrical properties between materials upon using it. In addition, it may cause problems such as high viscosity of the coating solution in terms of the preparation. The dispersion of the inorganic filler in (3) shows high abrasion resistance, as compared to that of the conventional photoconductor comprising a low molecular charge transporting material dispersed in inactive high molecules (polymer). However, traps on the surface of the inorganic filler tends to increase the residual potential, thereby causing reduction in the image density. Also, when unevenness of the inorganic filler and the binder resin on the surface of the photoconductor is severe, inferior cleaning may occur, resulting in toner peeling and image deletion. With these techniques (1), (2) and (3), it is impossible to satisfy sufficiently the durability required for organic photoconductors, including electrical durability and mechanical durability.
Furthermore, in order to improve the abrasion resistance and scratch resistance of (1), a photoconductor containing a cured body of a multi-functional acrylate monomer is disclosed (Japanese Patent No. 3262488). In this patent, the purpose of inclusion of cured material of this multi-functional acrylate monomer in a protective layer on the photoconductive layer is described; however, there is no more than a simple description that a charge transporting material may be contained in the protective layer and there is no concrete descriptions. Further, when a low molecular charge transport material is simply added to the surface layer, it may cause problems related with the compatibility to the cured body, thereby crystallization of the low molecular charge transporting material and clouding may occur, resulting in reduction in mechanical properties.
In addition, according to this photoconductor, since the monomer is reacted while it contains a polymer binder, the curing cannot be sufficiently progressed.
As technique for inhibiting abrasion of the photoconductive layer to substitute the above techniques, a process for forming a charge transporting layer using a coating solution comprising a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin (for example, Japanese Patent No. 3194392). The binder resin includes a binder reactive with the charge transport material having a carbon-carbon double bond and a binder non-reactive with the charge transport material without having the double bond. This photoconductor has attracted public attention since it shows abrasion resistance along with excellent electrical properties. However, when a non-reactive resin is used as the binder resin, the binder resin is poorly compatible with the cured body produced by the reaction of the monomer and the charge transporting material, thereby surface unevenness during cross-linking forms from the phase separation, resulting in cleaning failure. Also, as described above, in addition to the interference of the binder resin with the curing of the monomer, a bi-functional monomer which can be used in the photoconductor has a few functionality and fails to provide a sufficient cross-linkage density, thereby it is possible to obtain a sufficient abrasion resistance. Also, when a reactive binder is used, since the number of functional groups contained in the monomer and the binder resin is small, the bonding of the charge transporting material and the cross-linkage density cannot be satisfied at the same time and the electrical properties and abrasion resistance are not sufficient.
Also, a photoconductive layer containing a cured hole transporting compound having two or more chain polymerizable functional groups in a molecule (for example, JP-A No. 2000-66425).
However, according to the photoconductive layer, since the bulky transporting compound has two or more chain polymerizable functional groups, distortion may occur in the cured body, causing increase in internal stress, roughness of the surface layer, and formation of crack over the time.
Even in a photoconductor having a crosslinked photoconductive layer with a charge transporting structure attached in a chemical fashion, it cannot be said that general properties are sufficiently attained.
As explained above, even though the higher speed of apparatuses may be established through employing the photoconductors formed from titanyl phthalocyanines, the photoconductors must be often exchanged due to the decreased image quality caused by background smear; even though the abrasion resistance may be enhanced by forming a protective layer, the decrease of image quality may grow due to the increase of residual potential and inferior cleaning; as a result both of the higher sensitivity and prolonged life of the photoconductors desired for high-speed or color apparatuses have not been attained yet.
As such, image forming apparatuses with an improved photoconductor capable of forming images stably for a long period are definitely desired in the field of high-speed and color laser printers and digital copiers.